Piezoelectric transducers have been known and used for some time. Considerable work has been done in the last several decades, for example, for use in sonar systems. The use of such transducers for energization by electrical energy to produce a mechanical movement, as well as to receive mechanical input energy and produce a related electrical signal, is now well known. Such transducers have also been used in accelerometers, phonographs, cartridges, machine vibration sensors and/or counters, and related arrangements.
In the field of life and property protection, piezoelectric transducers have not been widely utilized as sensors or detectors. Instead vibration detectors such as spring-activated contact switches or mass loaded spring contacts have been employed. Other sensors, such as concentric free-floating masses which are relatively movable to function as a make-or-break switch, have also been employed, as well as coils moving in a magnetic field. However, for reasons of simplicity, economy, and reliability, as well as desired sensitivity over a certain frequency range, considerable work is still being directed to the improvement of such sensors for intrusion detectors.
It is, therefore, a principal object of the present invention to provide an improved sensor for intrusion detection systems, in which the sensitivity is significantly greater than has been attained with other mechanical devices.
Another important object of the invention is the provision of such an improved sensor which can protect an entire wall or wide area of an enclosure, responding to breakage of a window or to the opening of a door at a distance from the sensor itself.
Still another important object of the invention is to provide such a sensor which has a high sensitivity over a specific frequency range related to the type of intrusion against which protection is sought.
It is another important object of the invention to provide such an improved sensor which is highly responsive along two different axes of the usual three-axis coordinate system.